1. Technical Field
The present invention relates generally to a method of welding a coil end of a stator of an electric rotating machine such as an electric motor, an electric generator, or a motor-generator to be mounted in electric vehicles or hybrid vehicles.
2. Background Art
FIG. 1 illustrates a conventional stator 10 for use in electric rotating machines. The stator 10 includes an annular stator core 14 and a plurality of stator windings 16. The stator core 14, as illustrated in FIG. 2, has a plurality of slots 12 arrayed at regular intervals in a circumferential direction thereof. The stator windings 16 are wound through the slots 12 of the stator core 14. Each of the stator windings 16 is made up of a plurality of U-shaped conductors 16a inserted into the slots 12, for example, from the lower side of FIG. 1. Ends of the U-shaped conductors 16a extend outside an upper end, as viewed in FIG. 1, of the stator core 14 and are arrayed coaxially. Each of the U-shaped conductors 16a is twisted so that radially adjacent two ends 16b of the U-shaped conductors 16a extend in opposite circumferential directions of the stator core 14 to form a coil end pair 16c of the U-shaped conductors 16a which are to be welded together. The pairs 16c are, as can be seen from FIG. 1, arranged at regular intervals in the circumferential direction of the stator core 14 in coaxial circular lines and also aligned radially in a plurality of lines.
The coil ends 16b of each pair 16c may be joined by, for example, TIG (Tungsten Inert Gas) welding. This is achieved by inserting, as illustrated in FIG. 3, each negative electrode 18 of a TIG welding system in a direction Y1 from outside the circumference of the stator core 14 fully into a gap between every adjacent two of arrays of the coil ends 16b which extend in a radial direction of the stator core 14 and placing positive electrodes (not shown) from a direction perpendicular to the drawing (i.e., from above the coil ends 16b) close to the coil end pairs 16c. This is very efficient in that the arrays of the coil ends 16b which extend radially of the stator core 14 can be welded at once. However, if the distance L1 between radially adjacent two of the coil end pairs 16c along the surface of the end of the stator core 14 is not great enough to ensure electrical insulation therebetween, it may result in electrical discharge.
In order to prolong the distance L1, U.S. Pat. No. 6,181,044 B1 (corresponding to Japanese Patent First Publication No. 2000166150) filed on Nov. 16, 1999, assigned to the same assignee as that of this application, teaches a zigzag alignment of the coil end pairs 16c, as illustrated in FIG. 4. Specifically, an outer one 16c-1 of each radially adjacent two of the coil end pairs 16c is placed out of alignment with an inner one 16c-2 in the radial direction of the stator core 14. Such zigzag alignment results in increases in distance L2 between two of the coil end pairs 16c which are located adjacent each other in the radial direction of the stator core 14 and distance L3 between adjacent two of the coil end pairs 16c of the circumferentially extending arrays.
The zigzag arrangement of the coil end pairs 16c, however, encounters the drawback in that some of the coil end pairs 16c, as can be seen from FIG. 4, will obstruct the full insertion of the negative electrodes 18 of the TIG welding system from the direction Y1 into the gaps between the radially extending arrays of the coil ends 16b of the U-shaped conductors 16a. 